The present invention relates to a method and an apparatus for the recovery, in the form of a suspension in a liquid, of fibrils made of synthetic polymer which are in the presence of vapors of an organic solvent.
Various processes have been proposed which enable fibrils or fibrillated structures to be produced from synthetic polymers.
Thus, according to Belgian Pat. No. 568,524, issued June 11th, 1958 in the name of E. I. du Pont de Nemours, continuous structures consisting of a multitude of strands or fibrillar sections which gather together and separate at irregular intervals to form a "unitary fibrillar plexus" are produced by extruding a solution of a synthetic polymer, which is at a temperature above the normal boiling point of the solvent and under the autogenic pressure or under a higher pressure, through an orifice of suitable shape, into a zone of lower pressure.
According to this technique, the instantaneous vaporization of the solvent of the polymer solution at the instant when the pressure is suddenly released leads to the formation of continuous fibrillated structures which are discharged at very high speed and which is obviously accompanied by the formation of a very large volume of solvent vapors.
According to another technique, disclosed in Belgian Pat. No. 787,032 issued on Sept. 1st, 1972, discontinuous fibrils are produced by suddenly releasing the pressure acting on a two-phase liquid mixture of molten polymer and solvent which is at a high temperature and a high pressure, by passing it through an orifice, so as to bring about the instantaneous vaporization of the solvent and solidify the polymer, when an added fluid is introduced into the said two-phase liquid mixture before the release of pressure is complete. The added fluid can be identical to the solvent used to produce the two-phase liquid mixture. This second technique also causes a large volume of solvent vapors to be produced.
According to a third technique, disclosed in Belgian Pat. No. 787,033 issued on Sept. 1st, 1972, discontinuous fibrils are also produced by suddenly releasing the pressure acting on a two-phase liquid mixture of molten polymer and solvent which is at a high temperature and a high pressure so as to bring about the instantaneous vaporization of the solvent and to form a continuous fibrillated structure, and by shredding the continuous fibrillated structure thus produced, as it is formed, by means of a transverse stream of fluid.
The fluid forming the transverse stream can be identical to the solvent used to produce the two-phase liquid mixture. The production of discontinuous fibrils in accordance with this latter technique also leads to the evolution of large volumes of solvent vapors.
It is thus apparent that the various techniques for the production of fibrils made of synthetic polymer involve the use of very large quantities of an organic solvent which is vaporized completely at the last stage of the manufacturing process.
Consequently, in order especially to reduce the manufacturing costs, these solvent vapors have to be recovered so as to make it possible to recycle the solvent after condensation. Furthermore, this recovery is absolutely necessary and poses very significant problems when these solvent vapors are toxic or present the risk of forming explosive mixtures with oxygen.
There has also been proposed a process and an apparatus which make it possible to recover fibrils made of synthetic polymer which are in the presence of solvent vapors.
According to this process, the fibrils are recovered in the form of a web on a porous support which moves through a closed chamber containing vapors of a liquid which is not miscible with the organic solvent and is inert with respect to the polymer forming the fibrils, while the solvent vapors are extracted continuously from this chamber by suction. This web of fibrils, which is removed continuously from the closed chamber, possesses a certain internal cohesion and can be dried, wound up, stored and transported.
The fibrils of synthetic polymer of which the web is made are used most frequently as a starting material for the paper-making industry. They are processed like natural paper pulps prepared from wood. They thus undergo conventional operations of paper-making technique such as refining, lump disintegration, mixing and deposition on flat or round table machines. In order to do this, they must be employed in the form of a suspension in a liquid which is generally water. This suspension must be very homogenous and the fibrils therein must be finely divided. This is why the suspending of fibrils, starting from webs, is a long and delicate operation which ties up a large amount of equipment and is expensive in relation to the amount of energy consumed.